The present invention generally relates to rotating airfoil components of gas turbines and other turbomachinery. More particularly, this invention relates to turbine airfoil components having platforms configured to increase radial stiffness and reduce compressive stresses therein.
Buckets (blades) and nozzles (vanes) are examples of components that are located in the hot gas path within turbine sections of gas turbines. Whereas nozzles are static components, buckets are rotating components mounted to a rotor wheel within the turbine section to convert the thermal energy of the hot combustion gas to mechanical energy.
As a nonlimiting example, FIG. 1 schematically represents a bucket 10 of a land-based gas turbine engine of a type used in the power generation industry. As represented in FIG. 1, the bucket 10 comprises an airfoil 12 extending from a shank 14. The bucket 10 is further represented as being equipped with a dovetail 16 formed on its shank 14 by which the bucket 10 can be conventionally anchored to a rotor wheel (not shown) as a result of being received in a complementary slot defined in the circumference of the wheel. The dovetail 16 is conventionally configured to be of the “axial entry” type, in which the dovetail 16 has “fir tree” shape adapted to mate with a complementary-shaped dovetail slot in a rotor wheel. The airfoil 12 of the bucket 10 is directly subjected to the hot gas path within the turbine section of a gas turbine engine. The bucket 10 is also represented as having a platform 18 that forms a portion of the radially inward boundary of the hot gas path and, consequently, experiences very high thermal loads. Other relatively conventional features of the bucket 10 include sealing flanges (“angel wings”) 19 that project axially away from the forward and aft ends of the shank 14.
Buckets (and blades) of gas turbines are typically formed of nickel-, cobalt- or iron-base superalloys with desirable mechanical and environmental properties for turbine operating temperatures and conditions. Because the efficiency of a gas turbine is dependent on its operating temperatures, there is a demand for components that are capable of withstanding increasingly higher temperatures. As the maximum local temperature of a component approaches the melting temperature of its alloy, forced air cooling becomes necessary. For this reason, airfoils of gas turbine buckets often require complex cooling schemes in which air is forced through internal cooling passages within the airfoil and then discharged through cooling holes at the airfoil surface.
The high thermal loads to which the platform 18 is subjected are also detrimental to component life. In particular, high thermal loads can result in bulging and excessive deformation of the platform 18, leading to the possibility of low cycle fatigue (LCF) and creep failure. One form of platform deformation is schematically represented in FIG. 2, which shows a fragmentary view of the platform region of the bucket 10. As shown in FIG. 2, deformation of the platform 18 can result in a bulge 22 in the radially outermost (upper) surface 20 of the platform 18. The bulge 22 projects in a spanwise direction (indicated by an arrow in FIG. 2) of the bucket 10, which corresponds to the direction that the airfoil 12 extends from the platform 18. Because hot combustion gas flows across the platform surface 20 (also indicated by an arrow in FIG. 2), the bulge 22 can result in a downstream vortex that results in performance loss. To reduce thermal loads and their detrimental consequences, conventional practice is to employ a cooling scheme for the shank 14 and platform 18 of the bucket 10, often in the form of a cooling air flow obtained by air bled from the compressor section (not shown) of the engine. However, this purge flow is costly to the overall performance of a turbine engine, and therefore any reduction in the cooling air flow would be advantageous to turbine efficiency.
In view of the above, it would be desirable if the tendency and extent of deformation of bucket platforms could be reduced without requiring any further increase in cooling air flow.